1. Field of the Invention
This invention relates to navigation and communication systems and more particularly to method and apparatus for tactical navigation, communication and identification of a community of aircraft operating in a defined area.
2. Description of the Prior Art
There are several major types of aircraft navigation systems in use at the present time. Each of these types possesses inherent advantages and limitations which render that type suitable for particular applications. The so-called "inertial navigation systems" provide highly accurate aircraft attitude and acceleration information which enable the position of an aircraft to be located by a "dead reckoning" method of navigation. This system utilizes a gyroscopically stabilized platform having three axis accelerometers mounted thereon. The stable platform provides a set of reference axes to which the aircraft azimuth, pitch and roll are compared to provide measurements of the actual attitude of the aircraft. The velocity of the aircraft along each of the three axes are obtained by integrating the accelerometer output signals. When coordinates representing the initial or starting position of the aircraft are fed into the system, this arrangement provides excellent navigation information for aircraft navigation in a defined area of the earth's surface permitting the actual location of the aircraft to be determined without reference to other systems. However, the gyroscopically stabilized platform is subject to drift and other errors which render the system suitable for short term use only.
Radio navigation systems such as Loran, Tacan, and Omega supply georeferenced positional information to an aircraft so that the position of the aircraft on the earth's surface may be determined with a fair degree of accuracy. The Loran and Omega systems are so-called "hyperbolic systems" wherein the position of the aircraft along two intersecting hyperbolic lines is determined by measuring the time delay of transmitted radio signals from two pairs of stations located on the earth's surface. The Tacan system is a so-called "rho-theta system" which provides range and bearing information for the aircraft with respect to a plurality of radio beacons. Although systems of this type provide stable, long term georeferenced navigational information, the information supplied is of a lower order of accuracy for short term use than that obtained from inertial navigation systems. Furthermore, it is difficult to provide global coverage systems of this type without establishing an extensive chain of transmitting stations.
The third major type of navigation system is the so-called "satellite system". In this type of system, a plurality of radio wave transmitting satellites are caused to circle the earth at predetermined times and in predetermined orbits so that measurement of the radio waves from the satellites permits an aircraft or other space vehicle to determine its three dimensional position and velocity with respect to the earth's surface. Although this type of navigation system facilitates global coverage and provides position and velocity accuracy which is geometry sensitive and growth bounded, the short term accuracy of the navigational information supplied is of a lower order than that obtainable from the inertial systems or radio navigation systems. Additionally, a system user must be supplied with complex and expensive receiving equipment which renders the system prohibitively expensive for tactical navigation in a defined area.
From the foregoing discussion, it is believed apparent that none of the major types of navigational systems in present day use meets all of the requirements of a given application. Furthermore, an aircraft engaged in a tactical mission in a defined area on the earth's surface does not require the expensive and bulky equipment needed for satisfactory long range navigation, while an aircraft engaged in a long distance flight does not need the high degree of positional accuracy which an aircraft may require during takeoff or landing. In military and naval applications, "tactical" operations are often conducted by aircraft and ships in a limited or defined area on the earth's surface. During the course of these operations, many different aircraft and ships may be engaged in missions in the tactical area and consequently, positional and velocity information for each aircraft and ship in the operating "community" must be known with a high degree of accuracy to permit the close coordination of action required by modern military practice. For example, it is extremely important that aircraft in the tactical community engaged in reconnaissance and close air support for ground troops be furnished with navigational information of a high order which permits ground positions to be ascertained with great accuracy.
Similar requirements are also imposed upon aircraft and ships engaged in many civilian operations, such as search and rescue operations, for example. For maximum efficiency and thorough coverage of the area being searched, the position and velocity of each aircraft and ship in the tactical search area should be known with respect to every other aircraft and ship in the area. When the object of the search is located by an aircraft, it is important that the aircraft be able to ascertain and communicate the exact position of the object to the other members of the search group. In both military and civilian tactical operations, it is, of course, desirable that the command and control functions be capable of being exercised by one or more of the aircraft in the community. It is also desirable that the command and control functions be readily shifted from one aircraft or ship in the community to another aircraft or ship without loss of accuracy or stored data. In some applications, it is desirable that the members of the tactical community be identified as belonging to the community before being allowed to participate in the navigational data available. An example of such identification functions would be the military IFF ("identification friend or foe"). Finally, in military tactical operations it is extremely desirable that the overall operation be monitored by the area command unit without the emission by the command unit of electromagnetic radiation of any kind which would lead the enemy to the location of the command unit.
A suitable navigation system should, of course, provide the required navigational information with the required degree of accuracy, should be of compact size and weight, and should contain a minimum number of components. The system should also exhibit operating flexibility, be self-contained, and be capable of use in a wide variety of aircraft, ships and other vehicles. Additionally, the system design should be such as to permit maximum use of existing navigation technology and also be flexible enough to permit use of navigation technology developed in the future without making substantial changes in the overall system .